Add Sign API

[silvestrst-crypto]

I have implemented WCv2 Sign Api downstream, before WalletConnectRust project has started. https://docs.walletconnect.com/api/sign/overview

I have also implemented some of the Core APIs that are required to establish a simple WCv2 pairing and start communication. https://docs.walletconnect.com/api/core/pairing

The downstream proof-of-concept implementation allowed to successfully establish session and pairing, and exchange eth_signTypedData_v4 messages.

I would like to work on the Sign API for this project, and would like to enquire if anyone else started working on it - to avoid overlap.